In a connection of a conventional communication modem with a peripheral device through input and output ports, a parallel mode may be ideally used only in a relatively short distance. Accordingly, when the communication modem is spaced at a long distance from the peripheral device, it is difficult to connect the communication modem with the peripheral device through the parallel mode. Therefore, a serial interface mode is generally used in the connection of the communication modem to the peripheral device. It is determined whether the serial interface mode is able to be used according to the number of devices for transmitting and receiving data to/from the communication modem, a rate of data to be transmitted and received, a distance between the devices, and a possibility of transmitting and receiving the data simultaneously. The serial interface mode includes a universal asynchronous receiver/transmitter (UART), a serial peripheral interface (SPI) bus, an inter integrated circuit (I2C), and the like.